CMake: Use system C library for SDL from externals.

CMakeLists.txt

@@ -628,6 +628,7 @@ if(ENABLE_SDL)
     set(SDL_STATIC_ENABLED_BY_DEFAULT ON)
     set(SDL_TEST OFF)
     set(SDL_TEST_ENABLED_BY_DEFAULT OFF)
+    set(OPT_DEF_LIBC ON)
     add_subdirectory(Externals/SDL/SDL)
     if (TARGET SDL2)
       dolphin_disable_warnings_msvc(SDL2)